Dual-photon absorptiometry is being examined and developed for measurement of total body bone mineral in vivo. The method involves scanning with an external sealed source (1 to 2 Ci of 153-Gd). The transmitted radiation is detected with a scintillation detector. The source and detector are passed in a rectilinear raster pattern across the entire body during the course of about 70-minutes while the subject lies comfortably on the scanning table. These scan measurements permit the quantitation of the mass of mineral and the mass of soft-tissue at each point in the body; at points where there is no bone mineral the relative lean-fat composition of the soft-tissue is determined. We have acquired a graphics terminal and can now output the data in the form of quantitative images which show the distribution of fat, lean-tissue, and/or bone mineral. This also allows one to determine the composition of different anatomical areas of the body. This new technique is being evaluated in our laboratory. Factors affecting the precision and accuracy of measurement are being parameterized. Cross-validation studies are planned to assess how absorptiometric results compare with those obtained by partial- and total-body neutron activation analysis of calcium. Measurements are being made in a variety of patients, including those with osteoporosis, corticosteroid-induced osteopena, and renal disease. We are examining treatment of renal bone diseae with vitamin D metabolites. Measurements are also done of the 24-hour uptake of 99m-Tc diphosphonates using a whole-body shadow-shield detector. These studies complement the absorptiometric studies in the patients. We are planning to combine the emission- and transmission-scans to provide a bone image and to correct the emission scans for errors.